Overall Coefficient of Heat Transmission
The U-value, or U-factor (overall coefficient of heat transmission), represents the amount of heat (in Btu) that will pass through 1 square foot of a structural section per hour per degree Fahrenheit. This is the difference between the air on the inside and the air on the outside of the section. The U-value is the reciprocal of the sum of the thermal resistance values (R-values) of each element of the structural section. The total resistance (Rt) for the solid masonry wall illustrated in Table 3-2 is 3.47. Since the U-value is the reciprocal of Rt), the following is obtained:
11 U = R. = 347 = °.29
The overall coefficient of heat transmission (U-value or U — factor) is used in the formula for calculating heat loss or heat gain. The use of this formula is described in Chapter 4 of this book (“Heating Calculations”) and in Chapter 9 of Volume 3 (“Air Conditioning Calculations”).